1. Field of the Invention
This invention relates to an image signal restoring apparatus for restoring digitized image signals.
2. Description of the Related Art
In the conventional digital VTR, when recording an image signal on a magnetic tape, the analog image signal VDb FIG. 1(a)) is converted into digital data DD (FIG. 1(b)) of, for example, 8 bits by sampling and quantizing. To avoid occurrence of a harmful effect due to burst error, shuffling or the like dispersion treatment is then applied to this digital data DD. Further, as shown in FIG. 1(c), the digital data DD is grouped into blocks by every predetermined number of units. At its head, a block synchronizing signal SYNC is added, which, in view of the deshuffling (the treatment of returning the data to the original array), is followed by an address signal ADR. For detection and correction of a transmission error, an error detection code CRC and an error correction code parity P, Q, too, are provided following the data (DATA). Thus, formation of one synchronizing block SYNCB is completed.
A plurality of synchronizing blocks SYNCB are gathered up to form one track block TB (FIG. 1(d)), and this track block TB is magnetically recorded in one track of the magnetic tape.
Since the digital VTR performs recording in such a manner, many proposals for the high-efficient coding capability have been made in order to reduce the number of data recorded per unit time (compress the band) so that video signals of long time can be recorded. One of the high-efficient coding methods is to divide the entire area of a picture into (m.times.n) number of picture element blocks where m and n are integers of not less than 2. By utilizing the fact that the picture elements within one individual picture element block have high correlations, the coding is carried out in each picture element block. This is known as the so-called "block coding" method. In Japanese Laid-Open Patent Application No. Sho 62-151090, for example, the data of every one picture element block are converted into a form comprising the maximum and minimum values in that block and the data is obtained by quantizing each of all the picture elements between these values. Of the coding methods, this one by the block can be said, in the point of view of limiting the propagation of an error to within the block, to be suited to the digital VTR.
The block-coded data of a plurality of picture element blocks form the above-described synchronizing block to be recorded on the magnetic tape.
By the way, for reproduction of the image signal recorded on the magnetic tape, it is usual to prepare, besides the normal reproduction mode in which the recorded image signal is reproduced with fidelity, a high-speed search mode to be used in searching the recorded images at a high speed. In the normal reproduction mode, the recorded tracks of the magnetic tape are reproduced one at a time successively, and the signal reproduced from each track is written in the field memory. It is then read out in a predetermined periodicity and applied to a TV set or like monitor. In the high-speed search mode, on the other hand, the recorded magnetic tape runs very fast (for example, 8 times) while simultaneously reproducing the recorded images. On the display, therefore, it must be made possible to exchange one reproduced image by another at the high speed.
Since, in this high-speed search mode, the reproducing head scans across a plurality of tracks T1 to TN like a locus HTR shown in FIG. 2, there are many occasions that all the data necessary to reproduce the image cannot be picked up. Even if the reproduction of the image data DATA could be performed, the failure of reproducing the address signal ADR results in the impossibility of restructuring the image data DATA of that synchronizing block SYNCB. So, it has been the common practice in the art that in the high-speed search mode not only the data obtained in a time corresponding to one field (which are assigned to the hatched parts RSH of the picture elements on the picture DES shown in FIG. 3) but also the data obtained before that are used to form an image signal for one field.
But, even this method, because of the dispersion treatment such as shuffling, leaves an opportunity for some picture elements on the display to continue displaying the same content as it is not renewed for a long time. In this case, one displayed picture is made by mixing the very old data in some elements and new data in the others. Thus, an unnatural picture would appear. Particularly in the case of motion pictures, this phenomenon is conspicuous.
Also, there would occur a particular relationship between the shuffling method and the running speed of the tape that despite the use of the old data, some picture elements are never reproduced. This takes the form of a noise bar or the like on the display, causing the reproduced picture to become unpleasant to view. This tendency becomes more conspicuous particularly when the high speed search mode is operated, or in the case of the recording method using the above-described block coding.